The present invention relates generally to heaters for raising the temperature of a gas flow and, more particularly, to a duct burner for efficiently heating turbine exhaust gases in a less polluting manner.
It is well known to use gas burners for raising the temperature of turbine exhaust gas (TEG) sufficiently (typically by between 100°–600° F.) so that the TEG can be used to generate steam, for example. Generating steam with TEG is efficient because the energy that would otherwise be needed for reaching the temperature of the incoming TEG is saved. In the past, a variety of TEG heaters have been proposed, such as those disclosed in U.S. Pat. Nos. 6,301,875, 4,767,319, 4,737,100, and 4,462,795, for example.
A recurring problem with known TEG heaters is that they release pollutants, particularly CO. Significant amounts of CO are a byproduct of known TEG heaters because there is insufficient time to convert initially formed CO from combusting the heating gas into CO2 during the former's residence time in the flame or combustion zone of the heater. As part of the overall effort to protect the environment, regulations have therefore been promulgated in the United States which now limit the release of CO from TEG heaters to 0.1 lb/million btu generated by the heater. This is a stringent requirement in and of itself. It has become more difficult to attain with increased turbine efficiencies, which resulted in a decrease in O2 concentration (by volume) in the TEG. To alleviate this, it has been proposed to augment the TEG heater with additional air. Although this helps to reduce CO emissions, since more O2 is made available to effect a complete combustion of the heating gas, it lowers the efficiency of the heater because the augmenting air must be heated from ambient to the temperature of the incoming TEG.
Achieving complete combustion of the CO generated by the TEG heater becomes still more difficult when steam is injected into the turbine, which in turn reduces the O2 concentration in the TEG.
It has previously been recognized that CO emissions are reduced by increasing the residence time for the CO in the combustion zone of the TEG heater because this enhances the likelihood that CO will find an available O2 molecule and be converted to CO2. Thus, for several years a TEG heater has been in use which consisted of a flame shield that extended across the TEG duct, had a gas supply pipe positioned on a center line of the duct, and had a flame shield defined by plates which diverged (in the downstream direction) from the gas pipe towards the walls of the duct. Spaced-apart slits were arranged in the plate through which TEG could flow into the combustion zone located downstream of the flame shield. Diverging heating gas jets were injected into the combustion zone to generate turbulence and effect a better mixing of heating gas with the TEG. Although this TEG heater worked well, it is unable to meet today's tightened CO emissions standards.
Other known TEG heaters have attempted a variety of different approaches to reduce CO emissions. These attempts principally concentrated on efforts to discharge the heating gas into the TEG flow to maximize turbulence and thereby a mixing of the TEG with the heating gas and/or augmenting the TEG with air to provide greater O2 concentrations for oxidizing the heating gas. Still, the desired reduction in CO emissions to no more than 0.1 lb/106 btu in an energy efficient manner became difficult to attain.